1. Field of the Invention
The present invention relates to a direct sequence spread spectrum (DS-SS) communication system used for multiple access communications, such as satellite communications, cellular mobile communications, indoor wireless communications for computer remote terminal, and wireless local area network (LAN) communications.
2. Description of the Related Art
One application of DS-SS is code division multiple access (CDMA) technique, which is considered to be a candidate multiple access method for future telecommunication systems. In CDMA system, different kinds of pseudo-random noise (PN) codes and the same carrier frequency are assigned to each communication station. Since all stations use the same frequency band, each station shall be distinguished with the assigned PN code by using the property that the cross correlation among PN codes is very low.
In practice, according to the conventional CDMA communication system, information signals are spectrum spread by different PN codes first, and then the spectrum spread signals modulate carriers having the same frequency f.sub.a at respective transmit stations. Thus modulated signals are transmitted from respective transmit stations to receive stations after filtered by respective band pass filters. At one of the receive stations, the transmitted signals are frequency-converted by a local oscillator, and then despread by the same PN code used for spectrum spreading of desired information. Since PN code indicates strong correlation only with the signal spread by the same PN code, the desired information signal can be selectively obtained among the various received signals.
As shown in FIG. 1, according to the conventional CDMA communication system, spectrums of received signal from the transmit stations 1 to k (k is the number of transmit stations communicating simultaneously) are completely overlapped which each other because their center (carrier) frequencies coincide at the frequency f.sub.a.
Generally, as for PN codes for signal spectrums spreading in the DS-SS communication system, maximum length sequences are utilized. The number of existing maximum length sequences is small when the code length thereof is short, for example, the number is only 18 when the code length thereof is 127 bits, 16 when 255 bits, and 48 when 511 bits. Therefore, in order to accommodate more stations simultaneously in the CDMA communication system in which each of the transmit stations is distinguished only with the assigned PN code, maximum length sequences with extremely longer code length, in other words maximum length sequences having greater number of independent sequences would be required. The longer code length of the PN code sequences, the more complicating PN code generation process, construction of correlators, and also demodulation process.
In the CDMA system, the same PN code cannot be used at the same carrier frequency. Thus, according to the conventional CDMA system, in case that more transmit stations than the number of the existing PN codes are required in the system, a plurality of frequency bands should be employed to reuse the PN code as shown in FIG. 1. In this figure, one set of 1 to k stations with the carrier frequency f.sub.a and the other set of 1 to k stations with the carrier frequency f.sub.b are illustrated. However, this configuration requires m times of frequency band width in order to accommodate m times of the transmit stations simultaneously, namely to reuse the same PN code for m times.
Furthermore, in case that PN codes are not assigned permanently to the respective stations but assigned corresponding to the demands for communication, it is necessary that each station has a correlator device which can recognize all the PN code sequences used therein. This results in too many matched filters whose code patterns are physically fixed such as surface acoustic wave (SAW) filters, or the device which has the programmable functions. A digital logic LSI which can respond to various PN code sequences may be used. However, in case of using the PN codes with very long code length, information bit rate or operation speed thereof will be limited due to the heating or power consumption problems.